Professor Challenger, The Earth Core and The Moho

A photographic montage featuring a classic Joseph Clement Coll's Professor Challenger character seen running and waving an umbrella against a background of serpentinite. Image: NaturPhilosophieProfessor Challenger, We Meet At Last!

The days of Professor Challenger are here.  This week, scientists have set out to drill deeper into the Earth’s mantle than has ever been done before.  This time, let us hope the World will not scream! 

Scientists have set out this week to drill a hole into the Indian Ocean floor to try to get below the Earth‘s crust for the first time.  Their aim is to sample rock from the planet’s mantle – its deep interior.

The project is run under the auspices of the International Ocean Discovery Program (IODP).  Ultimately, it would give scientists access to fresh, unaltered peridotite – the rock, rich in olivine minerals, that, because of the size of the mantle, makes up the bulk material of the planet’s interior.  In the process, the researchers hope to check their assumptions about the materials from which Earth’s crust itself is made.

 

All Aboard!! The JOIDES Resolution

A photograph of the IODP Research Vessel - Joides Resolution at sea.
JOIDES Resolution – The IODP Research Vessel  Source: IODP

The “JOIDES Resolution” is a seagoing research vessel that drills core samples and collects measurements from under the ocean floor, giving scientists a glimpse into Earth’s development.  It is named after  HMS Resolution, commanded by Captain James Cook over 200 years ago, which explored the Pacific Ocean, its islands, and the Antarctic region.  Just like its namesake, the purpose of the current ship Resolution is to sail for scientific exploration.  This time, those discoveries lie beneath the ocean floor.  This is in addition to the 700m of water between the drilling ship – the JOIDES Resolution – and the seabed.

A map showing the location of the drilling site at Atlantis Bank, Indian Ocean.
The site for drilling: Atlantis Bank in the Southern Indian Ocean.

Data from the JR‘s ocean drilling offer a scientific means of understanding climate and environmental change throughout a significant part of our planet’s history – a research subject, often termed Earth’s paleoclimate.

The JR’s core samples are the “smoking gun” in evaluating many historical events related to paleoclimate, changes in the solid earth and paleoclimate – like plate tectonics and the extinction of the dinosaurs.

Right now, you can follow Expedition 360: SW Indian Ridge Live.

As they sink deeper and deeper, the researchers want also to test current models for how the crust is constructed and constituted.  Prof MacLeod is particularly keen to probe the so-called Moho boundary.

 

Reaching the Moho Boundary

A diagram explaining the Mohorovicic discontinuity refraction of the P-wave.
The Moho boundary was first identified in 1909 by Croatian seismologist Andrija Mohorovičić, when he observed that seismograms from shallow-focus earthquakes had two sets of P-waves and S-waves, one that followed a direct path near the Earth’s surface and the other refracted by a high-velocity medium. Source: Wikipedia.org

The Mohorovičić discontinuity, usually referred to as the Moho, is the boundary between the Earth’s crust and the mantle, the famous “discontinuity” where seismic waves from earthquakes abruptly change speed.

The textbook explanation is that the Moho draws the line between the crust and the mantle: a demarcation between familiar igneous surface rocks – such as granites, basalts and gabbros – and those of the interior peridotites.

The Moho is pretty uniform everywhere across the ocean basins, and because of that everyone has assumed that the ocean crust is very uniform and therefore, by inference, very simple.

Prof MacLeod suspects the discontinuity could also describe in places the depth to which water has managed to penetrate into the peridotites to produce a different type of rock known as serpentinite.

Serpentinite

A photograph showing a slab of serpentinite.
Serpentinite is made from mantle rock, altered by water seeping through cracks in the seafloor. Source: University of Cardiff

Serpentinite is a rock composed of one or more serpentine group minerals.  Serpentine minerals are formed by serpentinization – a hydration and metamorphic transformation of ultramafic rock from the Earth’s mantle.  The mineral alteration is particularly important at the sea floor at tectonic plate boundaries.

Soil cover over serpentinite bedrock tends to be thin or absent.  Soil with serpentine is poor in calcium and other major plant nutrients, and rich in elements toxic to plants, such as chromium and nickel.  Nevertheless, it might also say something really quite profound about life on Earth, because the process of making serpentinite (serpentinisation) is a huge draw for microbes.

The hydrogen and methane, produced as the peridotites’ olivine minerals are altered in contact with water, can be metabolised by single-celled organisms.

If serpentinite is shown to be more extensive than current models accept, then it means assessments of the scale of the biology occurring inside the Earth has also been greatly underestimated.  If there is far more serpentinite down there, who knows how much of it has microbes in it?

 

Expeditions 360: SW Indian Ridge

Last Tuesday saw the IODP Expedition 360 team board the JOIDES Resolution in Colombo, Sri Lanka. The ship put to sea on 5th December 2015.

Activity aboard the ship never ceases.  Operations go on 24 hours a day.  The JR’s complement consists of up to 50 scientists and technicians, and 65 crew members:

  • The science party changes with every expedition depending on its objectives.  The crew consists of marine professionals and ocean drilling specialists, among others, and rotates about every other expedition.
  • The JR’s crew includes technicians, cooks, curators, the folks who operate the ship and the drilling process, and many more.  Crew members tend to alternate duty on every other expedition, for the most part.
The left map of the SW Indian Ridge shows Actual Gravity data measured by satellite. And on the right, is a map of the the Predicted Topography at this location.
Colour-shaded relief images of the Southwest Indian ridge  Source: ngdc.noaa.gov

Because Atlantis Bank has been drilled before, and its condition is known, fairly swift progress is expected on this first outing, with the hole being opened to a depth of perhaps 1.3 km below the seabed.

Assuming all goes well, the next opportunity to drop the hole further still may come in 2018. This would be dependent on funding and of a capable ship like the JR being available.

Expedition 360 can also be followed live on the web hereSchools and the public were being encouraged to contact and interact with the science team directly.

Given that the seismic signature of this material is essentially the same as crustal igneous rocks, there is no way of telling – other than to drill and sample everything between the seabed and the top few hundred metres of unadulterated mantle.

If proven correct, the more sophisticated Moho description would have a number of far-reaching consequences for our understanding of how the planet is put together.  Notably, it would mean the igneous ocean crust is far more variable in thickness and structure than previously recognised.

It could change the game completely!

If the Moho seismic boundary is actually an alteration boundary from water penetration into the mantle, it means we know a lot less about the ocean crust than we thought we did.

 

Towards the Earth’s Core

A diagram explaining how the three phases of the JOIDES Resolution drilling mission will proceed. Phase 1 - Ship: JOIDES Resolution Date: December 2015 - January 2016. Phase 2 $ ($Proposed$ )$ - Ship: JOIDES Resolution Date: Unscheduled. Phase 3 - Ship: Chikyu Date: Unscheduled.
Phase 3 of the IODP mission will attempt to drill beyond the Moho boundary for the first time.  Source: IODP

We just do not know what the Earth core is made of.

So far, several attempts have been made to drill into the mantle.  None has yet succeeded.

However, this latest effort may fare better, because faulting and erosion have already thinned the crust at the targeted drill site, known as Atlantis Bank on the South West Indian Ridge of the Indian Ocean. 

In total, it will take three expeditions.  If all goes to plan…  The science is approved and the scientists have the funding for this initial two-month investigation.  But they will need to come back and may not complete the task until the 2020s.

It will take years to drop the full 5 to 5.5 kilometres (in addition to the 700 metres of water between the drilling ship, the JOIDES Resolution, and the seabed).

The endeavouring spirit of Professor Challenger lives on…